The present invention relates to optical storage devices and more particularly to bulk holographic storage devices.
The bulk storage of holograms is achieved by creating within the material interferences between a beam of radiation known as the reference radiation which is generally a parallel beam, and a radiation beam known as the object beam which carries the information and is generally of large apertural angle. These two beams are incident upon the same face of the storage material.
In order to superimpose holograms within a volume, the optical axis of the object beam remains fixed, while the optical axis of the reference beam rotates so that the different, successive holograms formed in the material are easily distinguishable at the time of reconstruction. The storage materials used have a high sensitivity. However, they cannot have their recording facilities fully exploited because geometric limitations, due to the aperture of the object beam incident upon the material which restricts the angular displacement of the reference beam for recording, which means that the maximum variations in refractive index cannot in fact be achieved.